Neoplastic diseases, characterized by the proliferation of cells not subject to the normal control of cell growth, are a major cause of death in humans. Clinical experience in cancer chemotherapy has demonstrated that new and more effective drugs are desirable to treat these diseases. Such clinical experience has also demonstrated that drugs which disrupt the microtubule system of the cytoskeleton can be effective in inhibiting the proliferation of neoplastic cells.
The microtubule system of eucaryotic cells is a major component of the cytoskeleton and is in a dynamic state of assembly and disassembly; that is, heterodimers of tubulin are polymerized and form microtubule. Microtubules play a key role in the regulation of cell architecture, metabolism, and division. The dynamic state of microtubules is critical to their normal function. With respect to cell division, tubulin is polymerized into microtubules that form the mitotic spindle. The microtubules are then depolymerized when the mitotic spindle's use has been fulfilled. Accordingly, agents which disrupt the polymerization or depolymerization of microtubules, and thereby inhibit mitosis, comprise some of the most effective cancer chemotherapeutic agents in clinical use.
Such anti-mitotic agents or poisons may be classified into three groups on the basis of their molecular mechanism of action. The first group consists of agents, including colchicine and colcemid, which inhibit the formation of microtubules by sequestering tubulin. The second group consist of agents, including vinblastine and vincristine, which induce the formation of paracrystalline aggregates of tubulin.
Vinblastine and vincristine are well known anticancer drugs: their action of disrupting mitotic spindle microtubules preferentially inhibits hyperproliferative cells. The third group consists of agents, including taxol, which promotes the polymerization of tubulin and thus stabilizes microtubules.
However, merely having activity as an antimitotic poison does not guarantee efficacy against a tumor cell, and certainly not a tumor cell which exhibits a drug-resistant phenotype. Vinca alkaloids such as vinblastine and vincristine are effective against neoplastic cells and tumors, yet they lack activity against some drug-resistant tumors and cells. One basis for a neoplastic cell displaying drug resistance (DR) or multiple-drug resistance (MDR) is through the over-expression of P-glycoprotein. Compounds which are poor substrates for transport of P-glycoprotein should be useful in circumventing such a MDR phenotype.
Accordingly, the exhibition of the DR or MDR phenotype by many tumor cells and the clinically proven mode of action of anti-microtubule agents against neoplastic cells necessitates the development of anti-microtubule agents cytotoxic to non-drug resistant neoplastic cells as well as cytotoxic to neoplastic cells with a drug resistant phenotype.